“It's a Sicilian message. It means Luca Brasi sleeps with the fishes.”
About this quote
Tessio explains a grim delivery in The Godfather (1972): a fish wrapped in the bulletproof vest of the family's enforcer, Luca Brasi. It is a Sicilian message that Brasi has been killed, and 'sleeps with the fishes' entered everyday speech from this scene.